Self-injection-locked (SIL) radar is conventionally applied for detecting important vital signs of biological subject (e.g. respiration and heartbeat). The principle of detection is that SIL radar radiates a radio-frequency (RF) signal to a subject, and the RF signal reflected from the subject is injected into the SIL radar to bring the SIL radar to a SIL state and generate a SIL signal. The displacement of the subject affects the frequency of the RF signal to cause a Doppler effect, for this reason, the SIL signal contains Doppler shift component caused by the displacement of the subject, so that the displacement information of the subject can be obtained by analyzing the SIL signal of the SIL radar in principle. However, when the displacement of the subject is more than 1/10 operating wavelength, the frequency information of the subject cannot be determined accurately because of nonlinear distortion of the waveform caused by SIL phenomenon. No matter the subject's movement is large or small, the conventional SIL radar cannot detect the displacement of the subject based on the variation of the waveform. The conventional SIL radar is limited to detect the vibration frequency of the subject in small movement, so it is only applied for detecting the frequency of the weak vibration subject, e.g. respiration and heartbeat. In addition, the conventional SIL radar cannot even detect the frequency of the vibration when the movement of the subject is large.